Imported object into unity lighting not working correctly
Real-time lighting and toon shading are both very dependent on mesh geometry since smooth shading only applies to shading with vertex normals and not raytraced shadow calculation that only accounts for vertex positions. With toon shader you are going to need more subdivisions on curved surfaces. 

So now you have a 180 degree arc expressed in 5 edges. Try making the arc with 9 or 19 subdivisions and you will see the quality improve.